This invention relates to a charging device for use in an electrophotographic apparatus, and more particularly to a charging device which charges, with improved efficiency and uniformity, a predetermined limited area of an electrophotosensitive member by corona discharge where a picture image is to be formed. It also relates to a charging device in which, for the purpose of preventing a fogging phenomenon and an excessive increase in picture image density during development due to an excessively high density distribution of electrostatic charges in an area adjacent to the predetermined limited area of the electrophotosensitive member as well as preventing a non-developing phenomenon or an excessive decrease in picture image density during development due to an excessively low density distribution of electrostatic charges, means are provided so that electrostatic charges can be uniformly distributed over the predetermined limited area of the electrophotosensitive member.
As is well known, electrophotography comprises electrostatically charging a photoconductive insulating layer of an electrophotographic film, exposing the charged area to form an electrostatic latent image of a picture image, applying a toner to the electrostatic latent image for positive or reversal development, and fixing the toner for recording the picture image. It has, therefore, an advantage over the conventional photography using a film of silver-halide photosensitive material in that the photosensitivity does not appear before the electrophotographic film is charged. For this reason, the electrophotography finds various useful applications in various fields. For example, in the field of microfilm recording, a film formed by laminating a photoconductive transparent insulating layer on a conductive support is used as an electrophotosensitive member, and a picture image is recorded on a predetermined area of the film by the steps of charging, exposing, developing and fixing. The picture image thus formed is then, for example, projected together with non-recorded areas of the film, and, when so required, a new picture image is subsequently additionally recorded on a non-recorded area of the film. This manner of picture image recording has made possible useful utilization of microfilms for applications which were not possible with the conventional silver-halide photography.
An electrophotographic apparatus comprises charging, exposure, developing and fixing devices. In order to obtain better image quality, it is essential that the entire image frame area of the electrophotosensitive member be uniformly charged. However, this has not been so very easy because, on one hand, corona discharge is generally used for charging by the charging device, and, on the other hand, it has been conventional in the process of microfilm preparation to charge only a predetermined desired area of the electrophotosensitive member without changing the relative positions of the electrophotographic member and the charging device. The charging efficiency has also been unsatisfactory because only a very limited proportion of ions generated by corona discharge could be utilized. It is, therefore, an object of the present invention to provide a charging device which achieves uniform charging and uniform distribution of electrostatic charges over the entirety of the predetermined area of the electrophotosensitive member with improved efficiency.
Before describing the present invention in detail, a charging device commonly used heretofore in the art will be described with reference to FIG. 1. In the drawing, a corona wire 1 is encased in a shielding casing 2 of U-shaped cross section formed of a metal plate in view of the necessity for application of a high electric field for generating corona ions. This shielding casing 2 is grounded to act as a mating electrode disposed opposite the corona wire 1 for generating corona ions, and a high voltage is applied across the corona wire 1. The high electric field thus established causes ionization of the atomosphere near the corona wire 1 so that corona discharge occurs between the corona wire 1 and the shielding casing 2. A portion of the corona ions leak outside from an opening 2a of the shielding casing 2 and impinge against that portion of a film 3 which faces the opening 2a thereby charging this film portion. Since the shielding casing 2 terminates close to the film 3, however, a substantial portion of the corona ions are absorbed by the shielding casing 2 before they reach the opening 2a, leaving a very small amount of ions to be discharged from the opening 2a, resulting in a low charging efficiency.
Therefore, a high-voltage power source having a large capacity is required. Moreover, an electric force acts upon the corona ions migrating through the opening 2a toward the film 3 thereby attracting the corona ion toward the marginal edge of the opening 2a of the shielding casing 2 grounded to act as the mating electrode, and, consequently, the density of corona ions impinging against the film 3 becomes non-uniform, resulting in non-uniform charging.
It should be noted, however, that such non-uniform distribution of the corona ions has posed no problem in the conventional copiers and like machines because the electrophotosensitive member and the corona charging device have relative velocities.
As one of means for attaining uniform charging, a charging device has been proposed to in which a conductive member provided on the peripheral edge of the opening 2a is connected with an external bias power source or grounded via a resistor having a high resistance so as to apply a bias potential which is substantially equivalent to the charged potential of the electrophotosensitive member, thereby correcting the electric field distribution in the vicinity of the opening 2a. The proposed device is effective in correcting non-uniform charging due to the electric field disturbance caused by the lower end of the shielding casing around the opening 2a and is particularly effective in correcting non-uniform charging along the longitudinal edges of the corona wire. This device, however, is not satisfactory as it causes non-uniform charging in the direction perpendicular to the corona wire when the device is used to uniformly charge a very limited narrow area of a microfilm for formation of a picture image thereon, under the conditon in which the electrophotosensitive member and the charging section are stationary relative to each other. It also causes non-uniform charging as described above when the distance between the corona wire and the electrophotosensitive member is limited to be almost equivalent to the width of the opening due to the limitation in space. This device is further defective in that the amount of charges at the peripheral portion of a picture image area becomes less than that at the central portion thereof because the distance from the corona wire differs depending on the central and peripheral portions of the image area. This device is also defective in that it is extremely difficult to apply a bias potential required for complete charging under the condition in which the potential of the electrophotosensitive member and that at the periphery of the opening become substantially the same, because the potential at the periphery of the opening rises shortly after it is charged, thereby giving rise to a gap between it and the potential of the electrophotographic member of the potential of the electrophotosensitive member becomes high than that at the periphery of the opening in a certain time.
A charging device has been disclosed in U.S. Pat. No. 3,991,311 which attempts to overcome the problems caused by the non-coincidence between the rate of increase in the surface potential of the electrophotosensitive member and the rate of increase in the periphery potential of the opening and to control the latter potential at a desired level by connecting a capacitor or the like to the conductive member so that the potential applied to the conductive member can vary with the progress of the process of charging. But this device is also defective in that difficulty is encountered in setting the capacitance of the capacitor suitable for a particular electrophotosensitive member and that it gives rise to an increase in the cost. The inventors, therefore, have made researches and studies to find out a best form of the mating or grounded electrode for generating corona ions to attain uniform charging and found that the shielding casing is preferably replaced by a mating electrode formed of a thin conductive member in wire- or sheet-like form, and this electrode is grounded to provide a greater potential difference between it and the corona wire. In order to prevent a fogging phenomenon or an excessive increase in picture image density during development due to excessively densely distributed electrostatic charges and also preventing an image-missing phenomenon or an excessive decrease in picture image density during development due to excessively thinly distributed electrostatic charges during electrostatic charging with corona discharge, a mask in the form of a film is preferably provided on the electrophotosensitive-member side of the opening in a relation making intimate contact with or closely adjacent to the electrophotosensitive member. It would be further effective when another conductive member is provided on the peripheral edge of the opening and a grounded resistor havig a high resistance is connected to this conductive member, thereby applying an auxiliary bias potential to the conductive member.